Vacuum fluorescent displays (VFDs) are the dominant electronic display technology in current production automotive vehicles, due in part because of the vacuum fluorescent display's generally superior emissiveness, brightness, and environmental and mechanical ruggedness in comparison to its contemporary alternatives; namely, cathode ray tubes (CRTs) and liquid crystal displays (LCDs). As an example, LCDs are well known to be sensitive to temperature extremes frequently encountered within a vehicle while vacuum fluorescent displays are insensitive to such extremes. Vacuum fluorescent displays also operate at low voltages, avoiding both the cost of expensive power supplies and the perceived problems of radiation associated with CRTs. In addition, high brightness vacuum fluorescent displays are suitable for use in automotive "heads-up" displays which are currently entering the market.
As illustrated in FIG. 1, the current state-of-the-art conventional fixed format vacuum fluorescent display generally consists of a number of cathodes 12, a grid 14 and an array of pixels 10 which serve as anodes. The thermionic cathode 12 is typically coated with barium oxide and is directly heated to approximately 600.degree. C. to induce electron emission. Electron emission is further promoted by a positive potential applied to both the grid 14 and the pixels 10. Each pixel 10 is coated with a phosphor corresponding to the color and luminance desired of the particular vacuum fluorescent display. The intervening grid 14 serves to improve the uniformity of brightness of the illuminated elements across the display. The entire package is then evacuated.
A wide variety of colored phosphors have been investigated for use in vacuum fluorescent displays to provide a particularly colored luminance or, alternatively, to provide a multicolored display. However the luminous efficiency of many of these colored phosphors is relatively low. To date, essentially all of the displays utilizing multiple colored phosphors have been fixed format displays--that is, the displays are limited to a fixed configuration and are therefore limited in the type of information which can be provided to the viewer. Reconfigurable vacuum fluorescent displays have been manufactured but are severely limited in application due to very low brightness, limited range of colors and relatively high voltage (150 V) power supply requirements.
These limitations result from the fact that in a conventional reconfigurable display application, each phosphor dot is only powered for a short period of time (which is proportional to the reciprocal of the number of rows in the display). For example, a 400 row display operating at a refresh rate of 60 Hz (that is each row being electrically addressed 60 times per second) would have each pixel powered for a maximum of 42.times.10.sup.-6 seconds per cycle, which is a duty factor (that is the fraction of time of which an individual pixel is addressed) of only about 0.25%. To compensate for this very low duty factor, high drive voltages have been used to increase display brightness. However, operation at these high voltages significantly degrades the performance of some colored phosphors, since the phosphors will tend to decompose under the condition of high voltage electron bombardment.
In order to eliminate these performance limitations of the conventional reconfigurable vacuum fluorescent display, it is necessary to provide some form of local memory at each pixel so as to alleviate such adverse consequences, as low brightness, which would otherwise result. The most practical approach to incorporating this local memory capability is the use of a simple transistor select-and-hold circuit at each pixel. With such a circuit, referred to as a pixel switch circuit, the current flow through each pixel can be controlled. The use of thin film transistors to provide the select-and-hold function is known in the art. In particular, the use of polycrystalline silicon (polysilicon) thin film transistors fabricated on glass or oxidized silicon substrates have demonstrated the required performance parameters, such as high reliability. An example of such a thin film transistor is a P-channel metal-oxide semiconductor field-effect transistor (MOSFET).
The basic structure of such a pixel switch typically incorporates three transistors and one storage capacitor. Two select transistors operated in series, though one select transistor is sufficient under some circumstances, are operated with their gate voltages supplied by a particular row of the display. The source voltage for the first select transistor is supplied by a particular column of the display. When both the particular column and row voltages are "on", the select transistors conduct and pass the electrical charge from the column contact to the storage capacitor. In turn, the voltage on this storage capacitor turns on the driver (or "pass") transistor which allows current to flow from the phosphor to ground, resulting in light emission from the phosphor. Performance of the pixel switch circuit is determined by the capability of the individual components, in particular the driver transistor and the capacitor. The current flow from the phosphor, which determines the phosphor's level of light output, is directly limited by the capacity of the driver transistor to allow on-current to flow to ground. As a consequence, in the case of multicolor reconfigurable vacuum fluorescent displays where different color phosphors having different luminous efficiencies, and correspondingly different current requirements, lie on adjacent pixels, the optimal current flow of each phosphor will differ accordingly. This is particularly true when an attempt is made to achieve color balance, that is the relative control over the emissive wavelength by a phosphor, which can then allow the observer to perceive a wide variety of colors with only the use of two or three colors. In addition, even with the different current flow requirements, the total storage capacitance at each pixel--as determined by the combined capacitance of the driver transistor and the capacitor--should be identical to achieve identical addressing characteristics and charge retention time.
Therefore, it would be advantageous to provide a pixel switch circuit for an active matrix vacuum fluorescent display in which there is provided a driver transistor which can be tailored to match the optimal on-current flow from the phosphor according to the luminous efficiency of the phosphor. It would also be advantageous that such a driver transistor be a MOSFET device whose capacitance is compensated with a capacitor which is tailored to maintain an optimal storage capacitance for each pixel switch circuit regardless of the phosphor being controlled.